In Wilson County, N.C., for example, police track location data if it is "relevant and material" to an ongoing investigation — a much lower standard than probable cause. Police in Lincoln, Neb., obtain phones' GPS location without warrants, and police in Tucson, Ariz., sometimes obtain the numbers of all the cellphones in a particular location at a certain time, a practice known as a "tower dump."

Other agencies, such as in North Las Vegas, Nev., and Wichita, Kan., obtain a warrant based on probable cause before accessing cellphone location data.

The ACLU said the documents show "pervasive and frequent violations of Americans’ privacy rights."

“What we have learned is disturbing. The government should have to get a warrant before tracking cell phones. That is what is necessary to protect Americans’ privacy, and it is also what is required under the Constitution,” said Catherine Crump, staff attorney for the ACLU Speech, Privacy and Technology Project. “The fact that some law enforcement agencies do get warrants shows that a probable cause requirement is a completely reasonable and workable policy, allowing police to protect both public safety and privacy.”

The Supreme Court ruled in January that using a GPS device to track a person's car qualifies as a search under the Fourth Amendment. The ACLU notes that the effects of that decision on similar tracking, such as obtaining cellphone location data, have "yet to be seen."